The invention relates to a method for correcting the gain and phase imbalance in quadrature paths of a receiver.
In radio communication systems, different types of modulation schemes are employed to minimize the frequency spectrum necessary for communication and thereby maximize the call capacity of the radio communication system. The modulation schemes utilized usually involve converting the communication signal into discrete form, and the resultant modulated signal is typically of a reduced frequency spectrum.
One method of transmitting a communication signal in discrete form is through the use of quadrature modulation. In quadrature modulation, the binary data stream of the encoded communication signal is separated into bit pairs. Such bit pairs are utilized to cause phase shifts of the RF carrier signal in increments such as plus or minus π/4 radians or plus or minus 3π/4 radians, according to the values of the individual bit pairs of the encoded signal.
The phase shifts are effectuated by applying the binary data stream comprised of the bit pairs to a pair of mixer circuits. A sine component of a carrier signal is applied to an input of a first mixer circuit, and a cosine component of a carrier signal is applied to an input of a second mixer circuit. The sine and cosine components of the carrier signal are in a relative phase relationship of ninety degrees with one another, or phase quadrature. A quadrature generator is utilized to generate and apply the sine and cosine components of the carrier signal to the first and second mixer circuits of the pair of mixer circuits, respectively. This produces what is reffered to as in-phase “I” and quadrature “Q” signals. These I and Q signals are then filtered and gain adjusted and finally sent to a Digital Signal Processing chip to extract the communicated data.
There are two major sources of I and Q signal errors in this type of receiver. First, I and Q gain and phase errors result from the down conversion to base band or intermediate frequency IF cause by the mixing circuits. Second, frequency dependent I and Q gain and phase error variations result within the pass band of the channel filters. These types of errors are due to gain and phase mismatches between the quadrature receiver paths after down conversion (e.g. between the I and Q low pass filters and between the I and Q gain control blocks). Therefore the IQ errors that need to be calibrated and corrected are; a) IQ gain errors (combined systematic and frequency dependent), b) systematic IQ phase errors, and c) frequency dependent IQ phase errors.
The prior art has used higher tolerance components in an attempt to avoid phase and/or amplitude imbalances between the I and Q components. Such an approach has significant cost impact and may still not adequately address the problem. Other prior art approaches attempt to account for imbalances by estimating and removing these errors.
One such approach is described in U.S. Pat. No. 5,396,656 issued on Mar. 7, 1995, to Jasper et al., for a Method For Determining Desired Components Of Quadrature Modulated Signals. This is shown in Prior Art FIG. 1. Here, a closed loop feedback technique is used to continuously determine an error signal by updating estimates of an imbalance component until the magnitude of the error signal is negligible. This prior art circuit contains standard components such as an antenna 301, a local oscillator 302, an A/D converter 303, and a Digital Signal Processing chip 304. The DSP 304 includes mixing circuits 305 and 306 and a phase shifter 307. The signals are then summed by adder 308 and then low pass filtered by element 309. The signal is then sampled by sampler 310, where the magnitudes of the components are estimated and the imbalance of the I and Q signals are determined by elements 311–314. The final error correction process is then accomplished by the desired component determiner 315 used in conjunction with the DSP. The drawback of this technique is that all these feedback components (310–315) must be supplied in addition to the already required components found in I and Q receivers. This adversely effects the cost and complexity of the device. Further, even with all these extra circuit elements, adequate error compensation is not fully realized.
Thus, conventional I and Q correction circuits rely on providing additional components for the minimization of errors. Other corrective devices such as a separate PLL and VCO are too costly to provide additionally. Therefore a solution is required that takes into account all the above mentioned problems and limitations associated with quadrature imbalance correction circuits without requiring additional expensive circuitry.